(1) Field of the Invention
The present invention relates to a substantially pure fragment of an Avian herpesvirus genome containing the gene for A antigen precursor polypeptide pr47 or a subfragment thereof which is the unglycosylated precursor to A antigen (gp57-65). In particular the present invention relates to an about 2.35 kbp fragment or subfragment of DNA from a Marek's disease herpesvirus which contains the gene that encodes for A antigen precursor pr47.
(2) Prior Art
Marek's disease (MD) is a lymphoproliferative disease of chickens caused by Marek's disease herpesvirus (MDHV), which results in T-cell lymphomas and peripheral nerve demylination (Marek, J., Multiple Nervenetzuendung (Polyneuritis) bei Huchnern. Dtsch. Tieraerztl. Wochenschr. 15:417-421 (1907); Pappenheimer, A. M., L. C. Dunn, and V. Cane, J. Exp. Med. 49:63-86 (1929); and Pappenheimer, A. M., L. C. Dunn, and S. M. Seidlin, J. Exp. Med. 49:87-102 (1929)). The disease was a major cause of economic loss ($200 million/year) to the poultry industry until the early 1970's (Purchase, H. G., Beltsville Symp. Agri. Res. 1:63-81 (1977)), when a live vaccine was developed from the antigenically related apathogenic herpesvirus of turkey (HVT) (Okazaki, W., H. G. Purchase, and B. R. Burmester, Avian Dis. 14:413-429 (1970)).
The ability of HVT to protect chickens against infection by MDHV may be due to an antigenic relationship between HVT and MDHV. At least six antigenically active viral proteins are common between MDHV and HVT (Ikuta, K. S., S. Ueda, S. Kato, and K. Hirai, J. Gen Virol. 64:961-965 (1983); and Silva, R. F., and L. F. Lee, Virology 136-307-320 (1984)). One of these antigenic proteins, the prominent MDHV A antigen (MDHV-A), was the first MDHV-HVT common antigen to be characterized on a physical, chemical and molecular basis (Glaubiger, C., K. Nazerian, and L. F. Velicer., J. Virol. 45:1228-1234 (1983); Long, P. A., K. Y. Parveneh, and L. F. Velicer, J. Virol. 15:1182-1192 (1975); and Long, P. A., J. L. Clark, and L. F. Velicer, J. Virol. 15:1192-1201 (1975)). MDHV-A is a glycoprotein with an apparent molecular weight of 57-65,000 daltons referred to as (gp57-65) in its fully glycosylated form (Glaubiger, C., K. Nazerian, and L. F. Velicer, J. Virol. 45:1228-1234 (1983); and Isfort, R. J., R. A. Stringer, H. J. Kung, and L. F. Velicer, J. Virol. 57:464-474 (1986)). A combination of cell-free translation, pulse-chase, and tunicamycin inhibitor studies have shown that MDHV-A is synthesized as a 47,000 dalton precursor polypeptide that apparently undergoes cleavage to remove a small signal peptide (Isfort, R. J., R. A. Stringer, H. J. Kung, and L. F. Velicer, J. Virol. 57:464-474 (1986)). The resulting 44,000 dalton polypeptide undergoes glycosylation and secretion from the cell in a precisely programmed manner (Isfort, R. J., R. A. Stringer, H. J. Kung, and L. F. Velicer, J. Virol. 57:464-474 (1986)). While MDHV-A is primarily secreted from infected cells (Isfort, R. J., R. A. Stringer, H. J. Kung, and L. F. Velicer, J. Virol. 57:464-474 (1986)), there is mounting evidence that a small amount is also associated with the plasma membrane in a specific manner (Ikuta, K. S., S. Ueda, S. Kato, and K. Hirai, J. Gen. Virol. 64:2597-2610 (1983); Nazerian, K., J. Gen. Virol. 21:193-195 (1973)). Not only is its role in the immunoprevention by HVT still unclear, it could have a role in the pathogenesis of MD since it has been recently postulated to play an immunoevasive role in protecting virus-producing cells of the feather follicle epithelium (Isfort, R. J., R. A. Stringer, H. J. Kung, and L. F. Velicer, J. Virol. 57:464-474 (1986)). Furthermore any virus-encoded glycoprotein should be a candidate for causing the early stage immunosuppression that is reported to occur after MDHV infection and may be one of the key events that lead to neoplasia (Payne, L. W., Biology of Marek's disease virus and the herpesvirus of turkeys, pp. 347-431. In B. Roizman (ed.), The herpesviruses, vol. 1. Plenum Press (1982)); especially in view of the observation that various virus particles, either infectious or non-infectious, inhibit mitogenic responses (Wainberg, M. A., B. Beiss, and E. Israel, Avian Dis. 24:580-590 (1980)).